1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of material, and, more particularly, from a fibrous web being processed into a paper product on a papermaking machine.
2. Description of the Related Art
During the papermaking process, a fibrous web of cellulosic fibers is formed on a forming wire by depositing a fibrous slurry thereon in the forming section of a paper machine. A large amount of water is drained from the slurry in the forming section, after which the newly formed web is conducted to a press section. The press section includes a series of press nips, in which the fibrous web is subjected to compressive forces applied to remove water therefrom. The web finally is conducted to a drying section which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation to yield a paper product.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entering the dryer section. As the dryer drums are often heated from within by steam, costs associated with steam production can be substantial, especially when a large amount of water needs to be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long press nips of the shoe type has been found to be more advantageous than the use of nips formed by pairs of adjacent press rolls. This is because the web takes longer to pass through a long press nip than through one formed by press rolls. The longer the time a web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip which can be five to ten times longer in the machine direction than one formed between two press rolls is formed. Since the long nip is five to ten times longer than that in a conventional two-roll press, the so-called dwell time of the fibrous web in the long nip is correspondingly longer under the same level of pressure per square inch in pressing force used in a two-roll press. The result of this long nip technology has been a dramatic increase in dewatering of the fibrous web in the long nip when compared to conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 5,238,537. This belt is designed to protect the press fabric supporting, carrying and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Belts of the variety shown in U.S. Pat. No. 5,238,537 are made by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness on at least the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press. It is specifically this coating which must have a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and fibrous web.
The base fabric of the belt shown in U.S. Pat. No. 5,238,537 may be woven from monofilament yarns in a single- or multi-layer weave, and woven so as to be sufficiently open to allow the impregnating material to totally impregnate the weave. This eliminates the possibility of any voids forming in the final belt. Such voids may allow the lubrication used between the belt and shoe to pass through the belt and contaminate the press fabric or fabrics and fibrous web. The base fabric may be flat-woven, and subsequently seamed into endless form, or woven endless in tubular form.
When the impregnating material is cured to a solid condition, it is primarily bound to the base fabric by a mechanical interlock, wherein the cured impregnating material surrounds the yarns of the base fabric. In addition, there may be some chemical bonding or adhesion between the cured impregnating material and the material of the yarns of the base fabric.
Long nip press belts, such as that shown in U.S. Pat. No. 5,238,537, depending on the size requirements of the long nip presses on which they are installed, have lengths from roughly 13 to 35 feet (approximately 4 to 11 meters), measured longitudinally around their endless-loop forms, and widths from roughly 100 to 450 inches (approximately 250 to 1125 centimeters), measured transversely across those forms. It will be appreciated that the manufacture of such belts is complicated by the requirement that the base fabric be endless prior to its impregnation with a synthetic polymeric resin.
It is often desirable to provide the belt with a resin coating of some predetermined thickness on its outer surface as well as on its inner surface. By coating both sides of the belt, its woven base fabric will be closer to, if not coincident with, the neutral axis of bending of the belt. In such a circumstance, the internal stresses which arise when the belt is flexed on passing around a roll or the like on a paper machine will be less likely to cause the coating to delaminate from either side of the belt.
Moreover, when the outer surface of the belt has a resin coating of some predetermined thickness, it permits grooves, blind-drilled holes or other cavities or voids to be formed on that surface without exposing any part of the woven base fabric. These features provide for the temporary storage of water pressed from the web in the press nip. In fact, for some long nip press configurations the presence of some void volume, provided by grooves, blind-drilled holes or the like, on the outer surface of the belt is a necessity.
Long nip press belt having a plurality of grooves are known. For example, U.S. Pat. No. 4,946,731 to Dutt shows such a long nip press belt, which has a base fabric which includes, in at least one of the machine and cross-machine directions, a spun yarn of staple fibers. When the base fabric is coated with a polymeric resin material, individual staple fibers extend from the spun yarns outward into the surrounding coating material. Subsequently, machine-direction grooves are cut into the coating on the outer surface of the belt. The so-called land areas separating the grooves from one another are anchored to the belt by these staple fibers, which make them less susceptible to delamination.
Another example, U.S. Pat. No. 6,428,874 to McGahern et al. shows a resin-impregnated endless belt for a long nip press of the shoe type that has a base structure impregnated by a polymeric resin material which renders the belt impermeable to fluids, such as oil, water and air. The polymeric resin material forms layers on the inner and outer sides of the base structure. The inner layer is smooth, but the outer layer has primary grooves for the temporary storage of water pressed from a paper web. The primary grooves are separated by land areas which have secondary grooves extending there across to relieve stresses which give rise to flex fatigue and stress cracking.
Accordingly, shoe press belts which are constructed with a grooved surface offer many advantages over belts without grooves, e.g. improved water removal, improved sheet profile, improved felt conditioning and felt lifetime. But in a number of applications, particularly on a slower speed paper machine, the advantages of using a grooved belt are less clear. Specifically, in applications where the press exhibits an ingoing nip spray (especially in the case of an inverted press) it may be more advantageous to use blind drilled circular holes on the surface of the belt rather than the above-described grooved belts. That is, ingoing nip spray is caused when the press fabric enters the pressure nip. Water is pressed out of the web by the press roll and into the press fabric and subsequently into the grooves. Because the grooves are continuous through the length of the belt, water is sprayed at the ingoing and outgoing nip ends. Ingoing nip spray leads to a loss of void volume in the press fabric, resulting in reduced web dewatering.
The present invention provides a solution to this problem by providing a shoe press belt with a grooved surface wherein the length of a number of a grooves may not be continuous and may be less than the length of the arcuate pressure shoe of the long nip press. The area of the press nip associated with the highest nip pressure (and highest water removal) is prior to the nip exit. As the groove exits the nip, the groove opening may not be present at the nip entrance or the nip entrance may be blocked because the length of the groove is less than the length of the arcuate pressure shoe and thus less than the length of the pressure nip. Since the nip entrance is blocked (not vented to the atmosphere) ingoing nip spray is reduced or eliminated, and hydraulic pressure within the press fabric is increased resulting in effective water removal from the web as the groove segment in the belt surface exits the nip. Accordingly, the discontinuous grooves of the present invention reduce or eliminate ingoing nip spray and increase the efficiency of dewatering.
The grooves of the above-mentioned present belt may extend in a direction substantially parallel to the machine direction (MD). Alternatively, the grooves of the present belt may be oriented in the cross-machine direction (CD) of the belt surface, and may be continuous or discontinuous.